Method for diaphragm electrolysis of alkali metal halides

ABSTRACT

A method for diaphragm electrolysis of an alkali metal halides which comprises electrolyzing an alkali metal halide solution by passing an electric current through an anode compartment and a cathode compartment of an electrolytic cell containing the alkali metal halide solution with an ion-exchange membrane of a graft copolymer of a polyolefin main chain and a side chain composed mainly of dihydroxystyrene and grafted to the polyolefin main chain separating the anode compartment and the cathode compartment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of copending application, Ser. No. 540,788, filed Jan. 13, 1975, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for electrolyzing halides of monovalent alkali metals using an electrolytic diaphragm, and more specifically, to a method for electrolyzing halides of monovalent alkali metals using a cation exchange membrane composed of a graft copolymer of a polyolefin main chain to which a side chain composed mainly of a dihydroxystyrene is grafted.

2. Description of the Prior Art

In the method of electrolyzing an aqueous solution of an alkali metal halide in an electrolytic cell including an anode, a cathode, a diaphragm disposed between them for separating the electrolytic cell into an anode compartment and a cathode compartment, and means provided outside of the cell for passing an electric current between the anode and the cathode, the feeding of the aqueous solution of the alkali metal halide into the anode compartment and the subsequent passing of a current between both electrodes result in the conversion of the halogen ion to the halogen at the anode. The alkali metal ion moves to the cathode compartment via the diaphragm, and an alkali metal hydroxide and hydrogen gas are generated at the cathode. Previously, a porous diaphragm made of asbestos has frequently been used for this purpose. However, since an asbestos porous diaphragm does not possess selective permeability between positive and negative ions and between monovalent and polyvalent ions, a part of the hydroxide ion formed in the cathode compartment diffuses into the anode compartment through the diaphragm to cause a reduction in current efficiency. At the same time, very small amounts of divalent or higher cations such as iron, magnesium or calcium contained as impurities also move to the cathode compartment together with the alkali metal ion, and thus cannot be removed. Thus, in order to prevent diffusion of hydroxide ions into the anode compartment, a technique is employed of flowing a part of the anodic solution into the cathode compartment through the diaphragm. However, an enormous cost in the subsequent concentrating and purifying steps is required because the aqueous solution of the alkali metal hydroxide obtained in the cathode compartment contains a large quantity of the alkali metal halide and traces of polyvalent metal ions, and the concentration of the resulting alkali metal hydroxide cannot be increased.

It is known that the use of a cationic exchange membrane as the diaphragm obviates the above defect. If, a cation exchange membrane having an ideal selective permeability to monovalent cations is used as a diaphragm, alkali metal hydroxides in high concentrations can be obtained from the cathode compartment without involving the above difficulties because the cation exchange membrane does not permit the permeation of hydroxide ions, halogen ions and polyvalent metal ions such as iron or magnesium. However, it is very difficult in practice to produce membranes having an ideal permselectivity to monovalent cations. Conventional cation exchange membranes have proved to be not entirely feasible for one or more reasons. For example, these membranes cannot ensure sufficient current efficiency or sufficient purity or concentration of the product. Moreover, since the membranes are exposed to severe conditions, they do not have sufficient endurance for use for prolonged periods of time.

SUMMARY OF THE INVENTION

An object of this invention to provide a method for electrolyzing alkali metal halides using a diaphragm having excellent performance.

Another object of this invention is to provide an electrolytic diaphragm having high permselectivity to monovalent cations, mechanical strength and durability.

The present invention provides a method for electrolyzing alkali metal halides using a cation exchange membrane as a diaphragm, the cation exchange membrane being composed of a graft copolymer of a polyolefin main chain to which a side chain composed mainly of a dihydroxystyrene is grafted.

More specifically, the present invention provides a method for diaphragm electrolysis of an alkali metal halide which comprises electrolyzing an alkali metal halide solution by passing an electric current through an anode compartment and a cathode compartment of an electrolytic cell containing the alkali metal halide solution with an ion-exchange membrane of a graft copolymer of a polyolefin main chain and a side chain composed mainly of dihydroxystyrene and grafted to the polyolefin main chain separating the anode compartment and the cathode compartment.

DETAILED DESCRIPTION OF THE INVENTION

The graft copolymer used in this invention can be crosslinked or sulfonated, or both crosslinked and sulfonated.

The diaphragm used in this invention can be produced, for example, by the following method.

The polyolefin used in this invention can be aliphatic hydrocarbon polymers, especially those polymers of monomer units having 2 to 10 carbon atoms, such as polyethylene, polypropylene, poly-1-butene, poly-1-pentene, poly-1-nonene, poly-1-decene, poly-3-cyclopentyl-1-propene, poly-3-cyclohexyl-1-propene, poly-3-methyl-1-butene, poly-3-cyclohexyl-1-butene, poly-3-phenyl-1-butene, poly-3-methyl-1-pentene, poly-3-methyl-1-hexene, poly-4,4-dimethyl-1-hexene, poly-5-methyl-1-heptene, etc.; aromatic hydrocarbon polymers, especially polymers of compounds represented by the general formula (I) ##STR1## wherein R₁, R₂ and R₃ each represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and A is an aryl group (a) having the formula ##STR2## or (b) having the formula ##STR3## in which R₄, R₅, R₆, R'₄, R'₅ and R'₆ each represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, such as polystyrene, poly(α-methylstyrene), poly(tertbutylstyrene), poly(p-methylstyrene), poly(2,4-dimethylstyrene), poly(2,5-dimethylstyrene), poly(m-ethylstyrene), poly(p-ethylstyrene), poly(p-isopropylstyrene), poly(p-hexylstyrene), poly(p-octylstyrene), poly(p-decylstyrene), poly(p-methyl-α-methylstyrene) ##STR4## poly(α-vinylnaphthalene), poly(β-vinylnaphthalene), poly(α-isopropenylnaphthalene), poly(α-vinyl-4-methylnaphthalene), poly(1-vinyl-5-ethylnaphthalene), poly(1-vinyl-5,8-dimethylnaphthalene), etc.; alicyclic hydrocarbon polymers, especially polymers of a monomer represented by the general formula (II)

    r.sub.7 hc= chr.sub.8                                      (ii)

in which R₇ and R₈ each represents a hydrogen atom, an alkyl group having 1 to 10 carbon atoms or a cycloalkyl group having 3 to 10 carbon atoms, and at least one of R₇ and R₈ is a cycloalkyl group such as a cyclopropyl group, a cyclopentyl group, a cyclohexyl group, a bicyclo[2,2,1]heptyl group, etc., such as polyvinyl cyclohexane, poly(4-methyl-1-vinyl-cyclohexane ##STR5## poly(3-methyl-1-vinyl-cyclohexane ##STR6## poly(1-cyclohexyl-propene-1) ##STR7## etc.; or copolymers derived from two or more aliphatic, alicyclic or aromatic monomers that constitute the above previously described polymers. The polyolefin that makes up the main chain of the graft copolymer can have branched chains.

The range of the degree of polymerization of these polymers is such that the polymers are solid at normal temperatures (e.g., 20°-30° C.). The polymers can be used in various desired forms such as powders, granules, fibers or films. If a polymer in a film form is used, the product can be used directly as a diaphragm.

The dihydroxystyrene that constitutes the dihydroxystyrene side chain of the graft copolymer can be any isomer, or a mixture of these isomers. A suitable proportion of the dihydroxystyrene side chain is about 5 to 500% by weight, preferably 20 to 200% by weight, based on the polyolefin main chain. Instead of dihydroxystyrene, a diacyloxystyrene can be grafted to the polyolefin main chain with subsequent hydrolysis of the acyloxy groups.

Examples of diacyloxystyrenes are those having the formula ##STR8## wherein R' represents a hydrocarbon group having 1 to 20 carbon atoms, such as a straight or branched chain alkyl group, or a --CH₂)_(n) P group wherein P is a cycloalkyl group, an alkyl (or phenyl)-substituted-cycloalkyl group, an aryl group or an alkyl-substituted-aryl group and n is 0 or an integer such that the total number of carbon atoms in the --CH₂)_(n) P group is not more than 20. Suitable examples of the --OCOR' group which can be substituted on the styrene moiety and which may be the same or different are acetoxy, propionyloxy, butyryloxy, valeryloxy, caproyloxy, stearyloxy, isobutyryloxy, trimethylacetoxy groups and the like; cyclopentanecarboxy, cyclohexanecarboxy, cyclooctanecarboxy, 1-methylcyclohexanecarboxy, 2,2-dimethylcyclohexanecarboxy, 1-isopropylcyclohexanecarboxy, 1-phenylcyclohexanecarboxy, cyclohexylacetoxy, ω-cyclohexylpropionyloxy; benzoyloxy, phenylacetoxy, 2-methylphenylacetoxy, 2,3-dimethylbenzoyloxy, toluoyloxy, p-ethylbenzoyloxy, p-isopropylbenzoyloxy and the like. Specific examples of suitable diacyloxystyrenes are 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-diacetoxystyrene, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dipropionyloxystyrene, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dibutyryloxystyrene, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-dibenzoyloxystyrene and the like. Most generally, 3,4-diacetoxystyrene is used.

In order to reduce the water content of the graft copolymer containing the dihydroxystyrene side chain and to increase the current efficiency thereof, the graft copolymer can, if desired, be crosslinked using a difunctional compound reactive with phenolic hydroxy groups, a polyene compound having at least 2 polymerizable double bonds in the molecule, or an organic sulfonic acid compound.

Examples of suitable difunctional compounds are diepoxides, for example, aliphatic, aromatic and alicyclic diepoxides, such as ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, bisphenol A diglycidyl ether, cyclohexane diol diglycidyl ether, and epoxu resins; diisocyanates such as hexamethylene diisocyanate, tolylene diisocyanate, xylylene diisocyanate, diphenylmethane-4,4'-diisocyanate, or hexahydrotolylenediisocyanate; and acid dihalides such as adipoyl dichloride, terephthaloyl dichloride and hexahydroterephthaloyl dichloride. A suitable amount of the difunctional compound is about 0.01 to 0.5 equivalent per equivalent of the phenolic hydroxy group.

Examples of suitable organic sulfonic acid compounds are aliphatic or aromatic sulfonic acids having 1 to 20 carbon atoms or their alkyl esters, e.g., in which the alkyl moiety has 1 to 20 carbon atoms. Examples of aliphatic sulfonic acids which can be used are methane sulfonic acid, ethane sulfonic acid, propane sulfonic acid and the like. Examples of aromatic sulfonic acids which can be used are benzene sulfonic acid, p-toluene sulfonic acid, m-xylene-4-sulfonic acid, β-naphthalene sulfonic acid and the like.

When the graft copolymer is crosslinked with an organic sulfonic acid compound, the graft copolymer is immersed in a solution of the organic sulfonic acid compound, e.g., in water, methanol, ethanol, diethyl ether, tetrahydrofuran, acetone, methyl ethyl ketone and the like, and reacted by heating, or the copolymer is impregnated with this solution and reacted by heating. A suitable concentration of the organic sulfonic acid in the solution ranges from about 0.1 to 20% by weight. A suitable reaction temperature is generally about 100° to 150° C. The reaction time differs according to the end use of the copolymer, or the degree of crosslinking, but generally, a suitable reaction time is about 5 minutes to 5 hours or longer.

The polyene compound having at least two polymerizable double bonds in the molecule can, for example, be an aliphatic compound such as butadiene, isoprene, 1,4-pentadiene, 1,4-hexadiene, 1,5-hexadiene, 1,2-divinyl-cyclobutane, 1,7-octadiene, 1,6-heptadiene, 1,3-pentadiene, dimethylbutadiene, 3-methyl-1,3-butadiene, etc., an alicyclic compound containing double bonds in the ring or in a substituent such as 1-alkyl-1-cyclopentene, cyclopentadiene, 1-alkyl-1-cyclohexene, cyclohexadiene, 4-vinylcyclohexene, ethylidene norbornene, etc., and an aromatic compound containing unsaturated substituents such as divinylbenzene (o-, m-, p-isomers or mixtures thereof), dialkylbenzene (o-, m-, p-isomers or mixtures thereof), etc. Suitable aliphatic compounds are aliphatic hydrocarbons, and aliphatic esters, such as (diesters formed between unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, angelic acid, tiglic acid, vinylacetic acid, allylacetic acid, diallylacetic acid, oleic acid, linolenic acid, linoleic acid, etc. and dihydric alcohols such as ethylene glycol, propylene glycol, butene-2,3-diol, propane-1,3-diol, butane-1,3-diol, butane-1,4-diol, etc., diesters formed between dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, etc. and unsaturated alcohols such as methallyl alcohol, crotyl alcohol, methylvinylcarbinol, allylcarbinol, 4-pentenol, cis-3-hexene-1-o1, etc., diesters formed between the above described unsaturated carboxylic acids and the above described unsaturated alcohols, diesters formed between the above described unsaturated carboxylic acids and the above described unsaturated diols, and diesters formed between the above described unsaturated dicarboxylic acids and the above described unsaturated alcohols, and the like. These compounds have 4 to 20 carbon atoms. Preferred examples of the polyene compounds are divinylbenzenes (o-, m-, or p-isomers or mixtures of these isomers), isoprene, butadiene, cyclopentadiene, ethylidene norbornene, diol esters of acrylic acid or methacrylic acid, or divinyl esters of adipic acid. Of these compounds, divinylbenzenes and isoprene are particularly preferably used. All of the o-, m- and p-isomers of divinylbenzene can be used in this invention. Generally, a mixture of these isomers is used. Generally commercially available divinylbenzene sometimes contains about 45% by weight of ethylvinylbenzene, but this mixture can be used as such in the present invention.

A membrane composed of a graft copolymer of a polyolefin as a main chain and a side chain composed of dihydroxystyrene and the polyene compound grafted to the main chain, in which the polyolefin is crosslinked by the polyene compound of the side chain, can also be used as the diaphragm in accordance with this invention. In this copolymer, too, the above-described various polyolefins, and polyene compounds, and dihydroxystyrenes can be used as constituents of the copolymer. Irrespective of the method of introducing the polyene compound, the amount of the dihydroxystyrene and polyene compound is preferably about 5 to 500% by weight based on the polyolefin main chain. If the amount is, less than about 5%, the resulting copolymer has insufficient properties as an electrolytic diaphragm. On the other hand, if the amount is larger than about 500% by weight, the resulting graft copolymer has insufficient strength and softness, and becomes difficult to use.

The grafting ratio, or the rate of introduction (based on the polyolefin), of the polyene compound is generally about 0.5 to 100%. If the grafting ratio is less than about 0.5%, the degree of crosslinking is low, and no outstanding effect is obtained by incorporating the polyene compound. On the other hand, if the grafting ratio is more than about 100%, crosslinking becomes excessive, and the polymer generally tends to be hard, brittle and tearable, and tends to have a high electric resistance that makes the passage of electricity through the polymer difficult. In view of the ion transport number, electric resistance and strength of the membrane, an especially preferred grafting ratio is about 20 to 200% for the dihydroxystyrene, and about 2 to 50% for the polyene compound.

Preferably, the dihydroxystyrene: polyene weight ratio is about 200:1 to 1:1, especially 50:1 to 2:1.

Furthermore, an ion-exchange membrane of a sulfonated product of the graft copolymer, either crosslinked or uncrosslinked, can also be used as a diaphragm in this invention. In these graft copolymers, sulfonic acid groups are introduced mainly into the dihydroxystyrene portion, but can be introduced into the polyene compound portion grafted for crosslinking purposes.

The rate of introduction of sulfonic acid groups is not particularly limited, but generally, about 0.5 to 2 sulfonic acid groups can be introduced per unit of the dihydroxystyrene. A suitable thickness of the membrane composed of the graft copolymer or the sulfonated product thereof is about 0.05 to 0.5 mm (in a wet condition).

The membrane composed of the above graft copolymer or its sulfonated product can be those prepared by any desired method. For example, a membrane composed of the graft copolymer containing a dihydroxystyrene side chain can be prepared by subjecting a polyolefin film to ionizing radiation in vacuo, in air or in an inert gas such as nitrogen, and then immersed in a solution of a dihydroxystyrene monomer or a diacyloxystyrene monomer or a mixture of these (when the diacyloxystyrene monomer is used, the grafted diacyloxystyrene is hydrolyzed to convert the diacyloxystyrene to dihydroxystyrene). The membrane can also be obtained by immersing the polyolefin film in a solution of the styrene monomer, and applying ionizing radiation (when a diacyloxystyrene monomer is used, hydrolysis is carried out subsequently as described above). Alternatively, a powdery or granular polyolefin is used in the above process instead of the film-form polyolefin to obtain a powdery or granular graft copolymer, and such a copolymer can be fabricated into film form using any desired film-forming techniques such as press-forming or melt-extrusion. If desired, the grafted copolymer obtained in a film form is subjected to the above-described crosslinking treatment. The production of the graft copolymer and its sulfonated product is described in detail in U.S. patent application Ser. No. 540,468 filed Jan. 13, 1975 (corresponding to Japanese Pat. Application No. 120,470/1974).

The crosslinking treatment using the polyene compound can be performed easily by utilizing ionizing radiation as in the case of the graft copolymerization.

In order to obtain a graft copolymer in which a side chain composed of the dihydroxystyrene and the polyene compound is grafted to a polyolefin main chain and which is crosslinked with the polyene compound of the side chain, a solution containing both the styrene monomer and the polyene compound monomer is used in the above-described method for graft copolymerization using ionizing radiation, whereby the styrene and polyene compounds are both grafted to the polyolefin, and crosslinking occurs by the polyene compound so grafted.

The styrene compound and the polyene compound are used for the graft copolymerization reaction as solutions in organic solvents which uniformly dissolve the styrene compound and the polyene compound, but do not dissolve the polyolefin. Examples of suitable organic solvents are ketones such as acetone or methyl ethyl ketone, esters such as ethyl acetate or butyl acetate, alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol or butyl alcohol, ethers such as tetrahydrofuran, aromatic hydrocarbons such as benzene or toluene, aliphatic or alicyclic hydrocarbons such as n-heptane or cyclohexane, or a mixture thereof. Because these aliphatic or alicyclic hydrocarbons have high affinity for the hydrocarbon polymers, they swell the polymers and permit easy introduction of the monomer. Thus, the grafting reaction is accelerated, and the grafting becomes uniform. The amount of these hydrocarbons should be such that they do not dissolve the polymer at the reaction temperatures, and is determined according to the type of the polymer.

The concentration of the monomer in the reaction solution is not critical, but generally, a suitable concentration of the monomer is about 0.1 to 80% by weight, preferably 5 to 50% by weight, based on the solution.

When the monomeric mixture to be grafted contains an unsaturated compound such as ethyl vinylbenzene, for example, such an unsaturated compound is also graft copolymerized and contained in the side chain. The presence of unsaturated compounds other than diacyloxystyrenes, dihydroxystyrenes and polyene compounds, especially monounsaturated compounds other than those having a polymerization inhibiting action, for example, styrene, 1-hexene and acrylic acid esters in addition to ethyl vinylbenzene, does not adversely affect the reaction. However, if the amount of such an unsaturated compound is too large, the effect of the present invention is reduced. For practical purposes, therefore, the amount of such a compound can be about 30% by weight or less based on the total monomeric mixture.

The source of ionizing radiation can be γ-rays, X-rays, electron beams, α-rays, or mixtures of these. A suitable intensity, that is, dose, of the ionizing radiation is about 10³ to 10¹¹ rads per hour. With electron beams, doses of as high as 10⁹ to 10¹¹ rads per hour can be used. Although lower doses can be used, a long time is required to obtain the desired amount of irradiation. Furthermore, higher doses can also be used, but are not feasible because higher doses may result in the structural changes of the polyolefin, for example, excessive crosslinking, cleavage of the main chain, and deformation and breakage of the polymer by heat.

The use of electron beams generated from an electron beam accelerator is especially effective since high dose irradiation can be obtained within short periods of time. The total dose of ionizing radiation required for graft copolymerization is usually from about 10⁵ rads to 10¹⁰ rads.

The temperature employed for ionizing radiation must be one at which the polyolefin is not dissolved and deformed. In view of the life of the generated radicals (which is short at high temperatures), a feasible temperature generally ranges from about -100° C. to 40° C. There is no particular lower limit to this temperature except that arising due to economical and technical problems.

The graft copolymerization reaction temperature generally ranges from the temperature at which the reaction mixture is a liquid, to about 100° C. If the reaction temperature is too low, the time required for the reaction increases, and if the reaction temperature is too high, gellation or homopolymerization under heat tends to occur. A suitable temperature can be selected so that such difficulties do not occur. For practical purposes, temperatures of about 0° to 70° C. are suitable. Where the ionizing radiation is applied in air, the graft copolymerization is preferably carried out at a temperature of about 60° C. or more because the peroxide generated must be decomposed. Ionizing radiation in advance in air or in a stream of nitrogen is commercially advantageous. Suitable additional atmospheres which can be employed include helium, neon, argon, carbon dioxide, etc.

The resulting graft copolymer, if desired, is washed with an organic solvent, for example, an alcohol such as methanol, ethanol or propanol, a ketone such as acetone or methyl ethyl ketone, or an aromatic hydrocarbon such as benzene or toluene, or mixtures of these. Hydrolysis of graft copolymers containing a side chain comprising the diacyloxystyrene, similar to an ordinary hydrolysis of phenol esters, is much easier to perform than the hydrolysis of esters of primary alcohols, and can be carried out easily under mild conditions. Specifically, the graft copolymer is placed in a solution of an acid such as hydrochloric acid, sulfuric acid or an organic sulfonic acid such as ethanesulfonic acid, p-toluenesulfonic acid, benzenesulfonic acid, etc. or a base such as sodium hydroxide or ammonia as a catalyst in water or in a mixture of water and an organic water-soluble solvent (e.g., alcohols such as methanol, ethanol, etc.; ketones such as acetone, methyl ethyl ketone, etc.; and ethers such as tetrahydrofuran, dioxane, etc.) to hydrolyze the acyloxy groups of the side chain. Since the hydrolysis is primarily carried out in a heterogeneous system, it is preferably performed in a mixture of water and a water-soluble organic solvent such as an alcohol or ketone in order to increase the affinity between the substrate and the catalyst and also to dissolve the organic acid that is split off in the case of using an acidic catalyst. A suitable hydrolysis temperature is about 50° to 100° C. and a suitable amount of the catalyst is about 0.1 to 50% by weight, preferably 1 to 20% by weight, based on the graft copolymer.

In order to obtain membranes composed of a sulfonated product of the graft copolymer, either crosslinked or uncrosslinked, any known method for sulfonating phenols, e.g., as disclosed in Rodd's Chemistry of Carbon Compounds 2nd Ed. Edited by S. Cottey, Vol. III, Part A, p.374, Elsevier Pub. Co., Inc., 1971, or sulfonating polyvinylpyrrolidone, e.g., as disclosed in D.I. Packam, J. Chem. Soc., 1964, 2617, can be used.

For example, the sulfonation can be effected by sulfonating the graft copolymer with concentrated sulfuric acid, sulfuric anhydride, or chlorosulfonic acid, etc. in the presence or absence of a solvent. Examples of suitable solvents which can be used in this process are halogenated hydrocarbons such as chloroform or carbon tetrachloride, polar solvents such as pyridine or dimethylformamide, or solvents such as diethyl ether or dioxane. Catalysts such as silver sulfate can also be used in this process.

When concentrated sulfuric acid is used, the film is immersed in concentrated sulfuric acid, and allowed to react for about 1 hour to about 10 days at about 0° to 40° C. If heating to a temperature of about 60° C. is carried out, the treating time can be shortened. If the temperature is too high, the base polymer is attacked with the properties of the polymer being degraded. In order to achieve a mild reaction, up to about 80% by weight of a solvent such as acetic acid or dioxane can be used. When fuming sulfuric acid containing about 5 to 60% by weight of sulfuric anhydride is used, the film is suitably treated at room temperature for about 2 to 10 hours. If the reaction proceeds excessively, the base polymer is also attacked. Where chlorosulfonic acid is used, the graft copolymer is dissolved in a solvent such as chloroform, dioxane, carbon tetrachloride or a mixture of these solvents in a concentration of about 1 to 60% by weight, and reacted at about 0° to 60° C. for about 1 hour to 10 days. Then, the reaction product is washed with water. The reaction conditions such as the temperature, the type of reagent, the concentration, or the reaction time are controlled as required so that the proportion of sulfonic acid groups introduced becomes the desired value.

If the graft copolymer is treated with concentrated sulfuric acid at room temperature for about 10 hours, about one sulfonic acid group is introduced per dihydroxystyrene unit. When a strong sulfonating agent such as chlorosulfonic acid is used, the treatment of the polymer in a solution of chloroform or dioxane, etc. results in the introduction of about 2 sulfonic acid groups per dihydroxystyrene unit, and a membrane having a high ion-exchange capacity can be obtained.

Membranes composed of the above graft copolymers or their sulfonated products have reduced permeability to halogen ions, low electric resistance, high permselectivity to monovalent cations, high mechanical strength and high durability.

In the practice of the method of this invention, any type of electrolytic cell including an anode, a cathode and a cationic exchange membrane as hereinabove described and provided between the electrodes can be used. For example, a two-compartment electrolytic cell divided into an anode compartment and a cathode compartment by the cationic exchange membrane, a three-compartment electrolytic cell in which an anode compartment and an intermediate compartment are separated from each other by a non-selective diaphragm and the intermediate compartment and a cathode are partitioned by the above cationic exchange membrane, or a modified three-compartment or four-compartment electrolytic cell built up by providing another non-selective diaphragm in the cathode compartment of the above three-compartment electrolytic cell. The arrangement of the electrodes is also optional. For example, they are arranged perpendicularly, or obliquely in parallel to each other, or not in parallel to each other. However, unless there is a special reason to do otherwise, electrodes provided in parallel to each other are generally used. The mode of providing the cationic exchange membrane can also be varied just as in the case of the electrodes, but usually, the membrane is provided perpendicularly. Generally, an iron or iron-type electrode is used as the cathode, and a graphite or dimensionally stable electrode is used as the anode. The scope of this invention, however, is not limited and any appropriate selection of these embodiments can be made.

At the beginning of the electrolysis using the membranes composed of the above graft copolymers or the sulfonated products thereof, the hydrogen atoms of the hydroxy groups of the dihydroxystyrene side chain are inevitably replaced with alkali metal ions. In the above case, it is preferred to affirmatively replace the hydrogen atoms of the hydroxy groups of the dihydroxystyrene side chain with alkali metal ions prior to the practice of the electrolysis. The electrolysis does, of course, proceed without any affirmative treatments for replacement of the hydrogen atoms by alkali metal atoms. In this case, the voltage employed in the electrolysis often necessarily must be increased or the electrolysis sometimes does not smoothly proceed until the replacement of the hydrogen atoms by the alkali metal atoms, which inevitably occurs, has taken place. After the replacement has occurred, the electrolysis proceeds smoothly.

As a result of the ability to more easily control the electrolysis after the above-described replacement has occurred, prior to use of the membranes, they are preferably contacted with an aqueous solution of an alkali metal hydroxide such as sodium hydroxide, potassium hydroxide and lithium hydroxide having a concentration of about 0.1 to 12N, an aqueous solution of an alkali metal hydroxide at the concentration as described above, additionally containing an organic solvent which can be uniformly mixed with the above-described aqueous solution and which is also capable of swelling the membranes, or a solution of an alkali metal hydroxide, at the concentration as described above, dissolved in a lower aliphatic alcohol having 1 to 4 carbon atoms at a temperature of room temperature (e.g., about 20°-30° C.) to about 90° C. for a period of about 30 minutes to about 5 hours. Suitable examples of an organic solvents which can be employed include methanol, ethanol, dioxane, tetrahydrofuran, acetone, methyl ethyl ketone, etc. Further, suitable examples of lower aliphatic alcohols which can be employed include methanol, ethanol, etc.

When the membranes used are composed of sulfonated products containing sulfonic acid groups, the hydrogen atoms of the sulfonic acid groups similarly are also inevitably replaced by alkali metal ions at the beginning of the electrolysis or in the replacement treatment as set forth previously.

When membranes composed of graft copolymers, such as, for example, a membrane which is produced by grafting side chains composed mainly of a diacyloxystyrene on a polyolefin and hydrolyzing the resulting graft copolymer in the presence of an alkali metal hydroxide as a catalyst, are employed, the hydrogen atoms of the hydroxy groups of the hydroxystyrene side chain thus produced in the membranes is already replaced by alkali metal ions. Therefore, in such a case, no particular replacement treatment is needed and the use of such copolymers in membranes results in the ability for the electrolysis to proceed smoothly without any affirmative replacement treatment being employed.

An aqueous solution of an alkali metal halide to be electrolyzed such as NaCl, NaBr, NaI, KCl, KBr, KI, LiCl, LiBr, LiI, etc. is fed into the anode compartment or the intermediate compartment, and an electrolytic current is passed between the electrodes whereby an alkali metal hydroxide is formed in the cathode chamber. At the surface of the cathode, hydrogen gas is formed, and at the surface of the anode, halogen is generated.

The alkali metal halides that can be electrolyzed by the method of this invention are halides of monovalent alkali metals. A suitable concentration of the solution electrolyzed generally ranges from about 1% by weight up to a saturated solution. A suitable current density is about 1 to 40 A/dm².

When sodium chloride is electrolyzed using the cationic exchange membrane in accordance with this invention, the temperature of the electrolytic cell can range from room temperature to about 90° C., and a suitable current density ranges from about 5 to 30 A/dm². If, for example, sodium hydroxide in a concentration of 15% by weight is to be produced at a current density of 10 A/dm² using a cationic exchange membrane having a thickness of 0.15 mm, a current efficiency of 70 to 95% can be obtained, and the resulting sodium hydroxide solution contains sodium chloride in an amount of less than about 0.02% by weight.

The following Examples are given to further illustrate the present invention in detail but the invention is not to be construed as being limited thereby. Unless otherwise indicated, all parts, percents, ratios and the like are by weight.

EXAMPLE 1

An electrolytic cell made of poly(methyl methacrylate) was used which was partitioned by a diaphragm having an effective area of 10 cm² into an anode compartment having a volume of 7 cm³ and a cathode compartment having a volume of 3.5 cm³ and in which a graphite plate was used as an anode and a stainless steel plate was used as a cathode. A solution feed opening and discharge openings for the solution and gas were provided in each of the anode compartment and the cathode compartment. Brine which was saturated with sodium chloride at room temperature (i.e., about 20° to 30° C.) and in which magnesium chloride was dissolved in a concentration of 20 ppm was fed into the anode compartment at a rate of 15 ml/min., and a prescribed amount of a 0.1N aqueous solution of sodium hydroxide was circulated into the cathode at a rate of 5 ml/min. Both of these solutions were pre-heated by a coil immersed in a constant-temperature water tank, and the temperature inside the cell was maintained at 70° C. Current of a specific density was passed for a prescribed period of time, and then the catholyte was analyzed to examine the concentrations of sodium hydroxide, sodium chloride and magnesium hydroxide. On the basis of the concentrations, the current efficiency was calculated.

Separately, one leg of a glass H-type cell (diameter 10 mm, thickness 0.5 mm) was charged with a 0.1 mm thick polyethylene film washed thoroughly with acetone and the other leg was charged with a solution of a mixture of 3,4-diacetoxystyrene and divinylbenzene (containing 55% by weight of divinylbenzene with an m- to p-weight ratio of about 2:1, and the remainder being mainly ethylvinylbenzene, hereinafter the same) in a weight ratio (3,4-diacetoxystyrene : divinylbenzene) of 9:1 in two times its weight of a mixture of benzene and acetone (in a benzene : acetone volume ratio of 3:1). By repeating a freezing-melting procedure five times, the cell was thoroughly degassed in vacuo, and then heat-sealed. The monomer solution part was frozen, and sufficiently covered with a lead plate. While the entire H-type cell was being cooled at -30° C., electron beams in a dose of 30 Mrads were applied to the polyethylene film at an acceleration voltage of 1.5 MeV using an electron beam accelerator. After the irradiation, the monomer solution was transferred to the film-containing portion to dip the film in the solution and allow it to react for 24 hours at 25° C. After the reaction, the cell was opened. The film was withdrawn, and thoroughly washed with benzene and acetone.

The thus obtained film was then placed in a 100 ml of flask equipped with a cooling tube. Subsequently, 50 ml of a mixture of concentrated hydrochloric acid and methanol in a mixing ratio by weight of 1:4 was added, and the flask was heated for 30 minutes over a hot water bath. As a result of the calculation from the difference in the weight of this film before and after the hydrolysis treatment, the grafting ratio of 3,4-dihydroxystyrene per se was 73% by weight, and the total grafting ratio was 98% by weight.

The film thus produced was used as a diaphragm. Prior to the electrolysis, the film was subjected to a pre-treatment in which the film was immersed in a 0.5N sodium hydroxide aqueous solution at a temperature of 80° C. for a period of 2 hours. Subsequently, electrolysis was carried out while passing an electric current of 1.0A (10A/dm²) for 5 hours. It was found that the concentration of sodium hydroxide in the catholyte was 17.2% by weight, that the concentrations of sodium chloride and magnesium hydroxide were 0.02% by weight and 6 ppm, respectively, and that the current efficiency was 84.5%.

EXAMPLE 2

A graft copolymerization reaction was carried out on a polyethylene film having a thickness of 0.2 mm using a mixed solution of 3,4-dihydroxystyrene and divinylbenzene (containing 90% by weight of divinylbenzene) by means of ionizing radiation from electron beams to obtain a film having a grafting ratio of 3,4-dihydroxystyrene per se of 82 wt% and a total grafting ratio of 105 wt% (based on the polyethylene). The above-produced film was subjected to a pre-treatment in which the film was immersed in a mixture of a 1N sodium hydroxide solution comprising water and dioxane (1:1 by volume) at a temperature of 80° C. for a period of 2 hours.

Electrolysis was then carried out in the same manner as described in Example 1 using the thus treated polyethylene film as a diaphragm. In the above, a neutral porous diaphragm was provided to separate the diaphragm of the graft copolymer as described above in the anode compartment from the anode, and brine was supplied between the diaphragm of the graft copolymer and the neutral porous diaphragm. It was found that the concentration of sodium hydroxide in the catholyte was 19.2% by weight, that the concentration of sodium chloride was 0.005% by weight, and that the concentration of magnesium hydroxide was 2 ppm. Further, the current efficiency was found to be 91%.

EXAMPLE 3

Electrolysis was carried out in the same manner as described in Example 1 using a diaphragm obtained by graft-copolymerizing a polyethylene film having a thickness of 0.2 mm with a 3,4-dihydroxystyrene side chain at a grafting ratio of 95% by weight using ionizing radiation from electron beams. It was found that the concentration of sodium hydroxide in the catholyte was 15.5% by weight, that the concentration of sodium chloride was 0.01% by weight, and the concentration of magnesium hydroxide was 4 ppm. Further, the current efficiency was found to be 79%.

EXAMPLE 4

Electrolysis was carried out in the same manner as described in Example 1 using a diaphragm obtained by graft-copolymerizing a polypropylene film having a thickness of 0.2 mm with a 3,4-dihydroxystyrene side chain at a grafting ratio of 92% by weight using ionizing radiation from electron beams. It was found that the concentration of sodium hydroxide in the catholyte was 16.2% by weight, that the concentration of sodium chloride was 0.02% by weight, and that the concentration of magnesium hydroxide was 5 ppm. Further, the current efficiency was found to be 75%.

EXAMPLE 5

A 0.2 mm thick polyethylene film was placed in a glass ampoule, and a solution of a mixture of 3,4-diacetoxystyrene and divinylbenzene in a weight ratio (3,4-diacetoxystyrene : divinylbenzene) of 9:1 in 9 times its weight of a mixture of benzene and acetone in a benzene : acetone volume ratio of 3:1 was placed in the ampoule. The ampoule was sufficiently degassed in vacuo by repeating a freezing-melting procedure five times and then heat-sealed. Using a cobalt 60 source, γ-rays were applied to the ampoule at a dose of 1.1 × 10⁵ rads/hour for 24 hours. Then, the film was taken out of the ampoule, washed sufficiently with benzene and acetone to remove a by-product copolymer composed of 3,4-diacetoxystyrene and divinylbenzene. The grafted film was hydrolyzed in the same manner as described in Example 1. As a result of the calculation from the difference in the weight of this film before and after the hydrolysis, the grafting ratio of 3,4-diacetoxystyrene per se was 50 wt% and the total grafting ratio was 69 wt%.

Electrolysis was then carried out in the same manner as described in Example 1 using the thus obtained film as a diaphragm. It was found that the concentration of sodium hydroxide in the catholyte was 17.5% by weight, that the concentration of sodium chloride was 0.02% by weight, and that the concentration of magnesium hydroxide was 4 ppm. Further, the current efficiency was found to be 82%.

EXAMPLE 6

A grafting reaction and a hydrolysis were carried out on a polyethylene film having a thickness of 0.2 mm in the same manner as described in Example 1 except that using a cobalt 60 source, γ-rays were applied at a dose of 1.1 × 10⁵ rads/hour at room temperature (i.e., about 20° to 30° C.) for 24 hours instead of the electron beams, and that a monomeric solution of 3,4-dihydroxystyrene in 2 times its weight of a mixture of benzene and acetone (3:1 by volume) was used, to obtain a film in which a 3,4-dihydroxystyrene side chain was graft-copolymerized at a grafting ratio of 64% by weight (based on the polyethylene). The resulting film was further sulfonated with concentrated sulfuric acid at room temperature for 72 hours to obtain a cationic exchange membrane.

Electrolysis was then carried out in the same manner as described in Example 1 using the resulting cationic exchange membrane as a diaphragm. It was found that the concentration of sodium hydroxide in the catholyte was 17.2% by weight, that the concentration of sodium chloride was 0.02% by weight, and that the concentration of magnesium hydroxide was 6 ppm. The current efficiency was also found to be 84.5%.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. 

What is claimed is:
 1. A method for diaphragm electrolysis of an alkali metal halide which comprises electrolyzing an alkali metal halide solution by passing an electric current through an anode compartment and a cathode compartment of an electrolytic cell containing the alkali metal halide solution with an ion-exchange membrane of a graft copolymer of a polyolefin main chain and a side chain composed mainly of a dihydroxystyrene and grafted to said polyolefin main chain separating the anode compartment and the cathode compartment.
 2. The method of claim 1, wherein said graft copolymer is crosslinked.
 3. The method of claim 2, wherein said graft copolymer is crosslinked with a difunctional compound reactive with phenolic hydroxyl groups.
 4. The method of claim 3, wherein said difunctional compound is used in an amount of about 0.01 to 0.5 equivalent per equivalent of phenolic hydroxyl group.
 5. The method of claim 3, wherein said difunctional compound is a diepoxide, a diisocyanate or an acid dihalide.
 6. The method of claim 3, wherein said difunctional compound is ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, bisphenol A diglycidyl ether, cyclohexane diol diglycidyl ether, an epoxy resin, hexamethylene diisocyanate, tolylene diisocyanate, xylylene diisocyanate, diphenylmethane-4,4'-diisocyanate, hexahydrotolylene diisocyanate, adipoyl dichloride, terephthaloyl dichloride or hexahydroterephthaloyl dichloride.
 7. The method of claim 2, wherein said graft copolymer is crosslinked with an organic sulfonic acid compound.
 8. The method of claim 7, wherein said compound is divinylbenzene.
 9. The method of claim 7, wherein said compound is isoprene, butadiene, cyclopentadiene, ethylidene norbornene, a diol ester of acrylic acid or methacrylic acid, or a divinyl ester of adipic acid.
 10. The method of claim 2, wherein said graft copolymer is crosslinked with a polyene compound containing at least two polymerizable double bonds in the molecule.
 11. The method of claim 10, wherein the amount of said polyene compound is about 0.5 to 100% by weight based on the polyolefin main chain.
 12. The method of claim 1, including pre-treating said ion-exchange membrane with a solution of an alkali metal hydroxide prior to the electrolysis.
 13. The method of claim 12, wherein said pre-treating comprises contacting said ion-exchange membrane with an aqueous solution of an alkali metal hydroxide; an aqueous solution of an alkali metal hydroxide additionally containing an organic solvent capable of being uniformly mixed with said aqueous solution and capable of swelling said ion-exchange membrane; or a solution of an alkali metal hydroxide in a lower aliphatic alcohol.
 14. The method of claim 13, wherein the concentration of said alkali metal hydroxide in said alkali metal hydroxide solution is about 0.1 to 12N and the contacting of said ion-exchange membrane with said alkali metal hydroxide solution is conducted at a temperature of about room temperature to about 90° C. for a period of about 30 minutes to about 5 hours.
 15. The method of claim 1, wherein said polyolefin is an aliphatic polymer, an aromatic polymer or an alicyclic polymer.
 16. The method of claim 15, wherein said polyolefin is polyethylene.
 17. The method of claim 15, wherein said polyolefin is polypropylene.
 18. The method of claim 15, wherein said polyolefin is polystyrene.
 19. The method of claim 1, wherein said side chain comprises a dihydroxystyrene and a polyene compound containing at least two polymerizable double bonds.
 20. The method of claim 19, wherein the weight ratio of the dihydroxystyrene to the polyene compound is about 200:1 to 1:1.
 21. The method of claim 1, wherein said graft copolymer is sulfonated.
 22. The method of claim 21, wherein said graft copolymer contains about 0.5 to 2 sulfonic acid groups introduced per dihydroxystyrene unit.
 23. The method of claim 1, wherein the dihydroxystyrene content is about 5 to 500% by weight based on the polyolefin main chain. 